1. Technical Field
This invention is directed towards regenerative hydride heat pump system and process.
2. Discussion of the Invention
Conventional dual metal hydride heat pumps comprise canisters containing two chemically different hydrides, one canister operates over a relatively lower temperature range, and the other canister operates over a relatively higher temperature range. The first or lower temperature performing hydride cools greatly, when providing hydrogen to the second or higher temperature performing hydride, and therefore can be used as a heat sink for cooling a room. The second or higher temperature performing hydride, when heated by an external source of heat desorbs hydrogen, which is used as a hydrogen source to the lower temperature performing hydride. Since higher temperature performing hydride take up hydrogen when cooled and desorb the hydrogen when heated, the higher temperature performing hydride side is heat driven usually by a relatively inexpensive source of heat such as natural gas or waste heat.
If the lower and higher temperature performing hydride canisters are cycled merely by exhausting their heat to the environment, no heat is regenerated. Consequently some regeneration schemes thermally link two low temperature canisters to each other and two high temperature canisters to each other thereby halving the sensible heat requirements. Net heating and cooling is then required to bring the canisters to the next required cycling temperatures. Unfortunately regeneration schemes such as this have raised the coefficient of performance or COP of hydride heat pumps to only about 0.5 to 0.6, i.e. for every 1000 watts of heat delivered to the hydride heat pump only 500 to 600 watts of heat is removed at a lower temperature. If the system were able to regenerate all of the sensible heat, the COP would be almost 1.0. It is therefore desirable to regenerate more of the sensible heat in hydride heat pumps.
In air conditioning systems the efficiency of the apparatus is usually measured by its coefficient of performance or "COP". By the term "COP" as used herein is meant the ratio of heating or cooling work performed divided by the amount of power required to do the work. Since cooling is generally the primary object of heat pumps, many systems are rated on their cooling COPs.
U.S. Pat. No. 4,372,376 discloses a hydride heat pump system which regenerates heat by a rotating valve device for each hydride which causes the heat transfer fluid to be directed to a particular bed or beds. It is disclosed that in operation the system, with a cycle time of about 4 minutes, a total hydride weight of about 82 kg (180 lbs), an output of 14.6 kw (50,000 BTU/hr) is obtained with a COP of about 1.5.
U.S. Pat. No. 4,436,539 discloses a dual hydride heat pump driven by waste heat for air conditioning buses which requires at least two vessels containing the higher temperature performing hydride and at least two vessels containing the lower temperature performing hydride. Water is used as the heat transfer fluid. It is mentioned that such units would weigh 445 kg and have a cooling capacity of 24.6 kw (84,000 BTU/hr) and be comparable to the weight of conventional bus air conditioning units. Since waste heat from the bus is used as the source of heat, heat regeneration is not a primary concern in this system.
Cryogenic heat regenerative cooler systems for sorption refrigerators using a physical, as opposed to chemical or hydride, sorption system having a heating/cooling loop and an expansion valve, with methane as a refrigerant gas and charcoal as the adsorbent, are disclosed in articles entitled "High Efficiency Sorption Refrigerator Design", and, "Design and Component Test Performance of an Efficient 4 W, 130 K Sorption Refrigerator" in Advances In Cryogenic Engineering, Vol. 35, Plenum Press, New York, 1990. Desorption occurs at 4.46 MPa (646 psia), i.e. P.sub.H, and adsorption at 0.15 MPa (22 psia), i.e. P.sub.L, or a pressure ratio of about 30, i.e. P.sub.H /P.sub.L =30. Methane is expanded from 4.46 MPa to 0.15 MPa to achieve cooling below 130 K (-143.degree. C.). The sorbent is heated from 240 K (-33.degree. C.) to 600 K (327.degree. C.) to desorb the methane.